Security element having a colour-shifting thin-film structure

Abstract

The invention relates to a security element (20) for securing value documents, having a colour-shifting thin-film structure, which comprises an absorber layer (22), a reflection layer (24) and a dielectric spacer layer (28) arranged between the absorber layer and the reflection layer. According to the invention, an ultra-thin adhesion promoter layer (28), with a thickness between 1 nm and 10 nm, is arranged between the dielectric spacer layer (26) and the reflection layer (24).

Description

[0001]Security element with color-shifting thin-film structure The invention relates to a security element for securing valuable documents with a color-shifting thin-film structure, which comprises an absorber layer, a reflective layer, and a dielectric spacer layer arranged between the absorber layer and the reflective layer. Data carriers, such as valuable or identification documents, but also other valuable objects, such as branded goods, are often provided with security elements for security purposes. These elements allow verification of the authenticity of the data carrier and simultaneously serve as protection against unauthorized reproduction. The security elements can, for example, be in the form of a security thread embedded in a banknote, a cover film for a banknote with a hole, an applied security strip, a self-supporting transfer element, or even in the form of a feature area printed directly onto a valuable document.Security elements with a viewing-angle-dependent appearance play a special role in authenticity assurance, as they cannot be reproduced even with the most modern copying machines. For this purpose, the security elements are equipped with optically variable elements that convey a different image impression to the viewer at different viewing angles and, for example, display a different color or brightness impression and / or a different graphic motif depending on the viewing angle. For this purpose, security elements with multilayer thin-film elements are often used, whose color impression changes for the viewer with the viewing angle (hereinafter referred to as the color-shift effect). The color-shift effect in such thin-film elements is based on viewing-angle-dependent interference effects caused by multiple reflections in the various sublayers of the element.The path difference of the light reflected by the various layers depends on the optical thickness of a dielectric spacer layer, which determines the distance between a semitransparent absorber layer and a reflective layer, and also varies with the respective viewing angle. Since the path difference is on the order of magnitude of the wavelength of visible light, the extinction and amplification of certain wavelengths results in an angle-dependent color impression for the observer. By appropriately selecting the material and thickness of the layers involved in the interference effect, a variety of different color-shift effects can be created.However, known thin-film elements with a layer sequence consisting of an absorber layer, a dielectric spacer layer, and a reflective layer tend to have a high tendency to gap, depending on the thickness of the dielectric spacer layer and also depending on any embossing lacquer used and arranged beneath the layer sequence of the thin-film element. In particular, insufficient adhesion of the reflective layer material to the dielectric spacer layer can cause problems during production, but above all also with the circulation stability of the security elements equipped with the thin-film structures. Based on this, the invention is based on the object of specifying a security element of the type mentioned at the outset with improved gap protection and high circulation stability. This object is achieved by the features of the independent claims. Further developments of the invention are the subject of the subclaims.According to the invention, in a generic security element, an ultra-thin adhesion promoter layer with a thickness of between 1 nm and 10 nm is arranged between the dielectric spacer layer and the reflective layer. The inventors have surprisingly found that the adhesion between the dielectric spacer layer and the reflective layer is significantly improved by such an additional adhesion promoter layer, in particular a chromium layer or a titanium layer, and the problems associated with the tendency of conventional three-layer structures to crack can be overcome. At the same time, an ultra-thin adhesion promoter layer in the aforementioned thickness range has only a minor influence on the color effect and the color-shift effect of the thin-film structure. If necessary, this minor influence can be compensated for by adjusting the layer thickness of the dielectric spacer layer.In a preferred embodiment, the absorber layer is formed by a chromium layer, which advantageously has a layer thickness between 2 nm and 10 nm, for example of approximately 6 nm. The reflection layer is advantageously formed by an aluminum layer, in particular by an opaque aluminum layer. The aluminum layer can have a layer thickness between 5 nm and 200 nm, in particular between 30 nm and 100 nm. The dielectric spacer layer is advantageously formed by an SiO2 layer. The layer thickness of the dielectric spacer layer is selected such that a desired color shift effect is achieved, i.e. a first color impression when viewed vertically and a second, different color impression when viewed obliquely. The ultra-thin adhesion promoter layer is advantageously formed by a chromium layer or a titanium layer, preferably by a chromium or titanium layer with a thickness between 1 nm and 4 nm.Currently, the use of chromium for the adhesion promoter layer is particularly preferred. A layer thickness of 3 nm for the ultra-thin chromium or titanium layer has proven particularly advantageous. In a preferred design, the color-shifting thin-film structure represents a four-layer structure containing, in this order, an absorber layer made of chromium, a dielectric spacer layer made of SiO2, the aforementioned ultra-thin chromium layer, and a reflective layer made of aluminum. In an advantageous development of the invention, the color-shifting thin-film structure is applied to an embossed lacquer layer provided with a relief structure.The relief structure is advantageously a diffractive structure, in particular a hologram, a holographic grating image or a hologram-like diffraction structure, an achromatic structure, in particular a matte structure, or a micromirror arrangement or a blazed grating with a sawtooth-like groove profile or a Fresnel lens arrangement, or is formed by a combination of two or more of the aforementioned structures. The security element is preferably a security thread, a security strip, a patch, or a label. The invention also includes a data carrier, in particular a valuable document, with a security element of the type described.The data carrier can be, in particular, a valuable document, such as a banknote, in particular a paper banknote, a polymer banknote, or a composite film banknote, a share, a bond, a certificate, a voucher, a check, a high-value admission ticket, or even an identification card, such as a credit card, a bank card, a cash payment card, an authorization card, an identity card, or a passport personalization page. The security element can, in particular, be arranged in a window area of ​​the data carrier.The invention further includes a method for producing a security element of the type described, in which an absorber layer, a dielectric spacer layer, and a reflective layer are applied to a substrate, and in which an ultra-thin adhesion promoter layer with a thickness of between 1 nm and 10 nm is applied, in particular sputtered, between the dielectric spacer layer and the reflective layer. The substrate can in particular be formed by a carrier film or by a carrier film that has already been coated, for example, provided with an embossed embossing lacquer layer. Further exemplary embodiments and advantages of the invention are explained below with reference to the figures, in which a true-to-scale and true-to-proportion representation has been omitted in order to increase clarity. They show: Fig. 1 a schematic representation of a banknote with two security elements according to the invention, Fig.2 shows a security element according to an embodiment of the invention schematically in cross section, and Fig. 3 shows a security element according to a further embodiment of the invention, in which a color-shifting thin-film structure is combined with a relief structure. The invention will now be explained using the example of security elements for banknotes. Figure 1 shows a schematic representation of a banknote 10 provided with two security elements 12 and 14 according to the invention. The first security element is a security thread 12 applied to the surface of the banknote. The security thread can of course also be formed by a window security thread which protrudes at certain window areas on the surface of the banknote, while being embedded in the areas in between inside the banknote.The second security element is formed by an adhesively bonded transfer element 14 of any desired shape, which is arranged in a window region 16 of the banknote 10 and, for example, covers a through-opening of the banknote. However, the transfer element can also be arranged over an opaque or translucent material region of the banknote. The particular structure of security elements according to the invention, such as the security thread 12 or the transfer element 14, will now be explained in more detail with reference to Fig. 2, which schematically shows a cross-section of a security element 20 according to the invention. The security element 20 contains a four-layer color-shifting thin-film structure with an absorber layer 22 made of chromium, a reflective layer 24 made of aluminum, and a dielectric spacer layer 26 made of SiO2 arranged between the absorber layer 22 and the reflective layer 24.As a special feature, an ultra-thin chromium layer 28 with a thickness of between 1 nm and 10 nm, in particular between 1 nm and 4 nm, is arranged between the dielectric spacer layer 26 and the reflective layer 24 to improve adhesion, resulting in an overall four-layer structure. Surprisingly, it has been found that such an additional ultra-thin chromium layer 28 can significantly improve the adhesion between the spacer layer 26 and the reflective layer 24, and that the problems associated with the tendency to crack in conventional three-layer structures can be reduced or even completely overcome. A layer thickness of the ultra-thin chromium layer 28 of approximately 3 nm has proven particularly effective.The presence of the additional, to a certain extent light-absorbing chromium layer 28 slightly shifts the color effect of the original layer sequence comprising absorber 24, dielectric 26, and reflector 28. However, this shift can, if desired, be easily compensated for by adjusting the layer thickness of the dielectric spacer layer 26. This makes it possible to provide a thin-film structure according to the invention with the same color effect as a conventional thin-film structure, but with significantly improved gap protection. It is understood that the basic layer structure of Fig. 2 can be supplemented, depending on the application, with further, known layers, such as a protective layer, an adhesive layer, or an embossing lacquer layer. Instead of an ultra-thin chromium layer, an ultra-thin titanium layer can also be used to improve adhesion.Figure 3 shows a security element 30 according to a further embodiment of the invention, in which the color-shifting thin-film structure is combined with a relief structure, such as a micromirror arrangement. The security element 30 contains a carrier film 32, for example a transparent, colorless PET film, to which an embossing lacquer layer 34 has been applied and provided with a relief embossing 36. The relief embossing 36 can, for example, represent a micromirror arrangement for producing a desired optical effect. An absorber layer 22 made of chromium, a dielectric spacer layer 26 made of SiO2, an ultra-thin chromium layer 28 with a thickness between 1 nm and 10 nm, in particular between 1 nm and 4 nm, and a reflective layer 24 made of aluminum were then applied to the embossed embossing lacquer layer 34, as already described in principle in FIG. 2.Furthermore, a leveling lacquer layer 38 and further layers 40, for example a primer layer and an adhesive layer, are provided for applying the security element to a target substrate. The carrier film 32 can be removed after the security element has been applied to the target substrate, or alternatively, it can remain in the layer structure. In the former case, a release layer facilitating the detachment of the carrier film can additionally be provided between the carrier film 32 and the embossing lacquer layer 34. The security element 30 of Fig. 3 is designed for viewing from the side of the carrier film 32, since from this viewing direction, the reflective layer 24 represents the layer of the thin-film structure furthest away from the observer.If the view is to be taken from the opposite side of the security element, the layers of the thin-film structure can also be applied to the embossing lacquer layer 34 in the reverse order, i.e. in the order reflective layer 24, adhesion-improving ultra-thin chromium layer 28, dielectric spacer layer 26 and absorber layer 22. The leveling lacquer layer 38 and further layers 40 are then applied to the absorber layer 22. In this embodiment too, an ultra-thin titanium layer can be used to improve adhesion instead of an ultra-thin chromium layer. List of reference numerals Banknote Security thread Transfer element Window area Security element Absorber layer Reflective layer Dielectric spacer layer Ultra-thin chromium layer Security element Carrier foil Embossing lacquer layer Relief embossing Leveling lacquer layer Further layers.

Claims

Patent claims 1. A security element for securing valuable documents with a color-shifting thin-film structure comprising an absorber layer, a reflective layer, and a dielectric spacer layer arranged between the absorber layer and the reflective layer, characterized in that an ultra-thin adhesion promoter layer with a thickness of between 1 nm and 10 nm is arranged between the dielectric spacer layer and the reflective layer.

2. A security element according to claim 1, characterized in that the absorber layer is formed by a chromium layer.

3. A security element according to claim 1 or 2, characterized in that the reflective layer is formed by an aluminum layer.

4. A security element according to at least one of claims 1 to 3, characterized in that the dielectric spacer layer is formed by an SiO2 layer. 5.Security element according to at least one of claims 1 to 4, characterized in that the ultra-thin adhesion promoter layer is formed by a chromium layer or a titanium layer, preferably by a chromium or titanium layer with a thickness between 1 nm and 4 nm.

6. Security element according to claim 5, characterized in that the ultra-thin chromium or titanium layer has a thickness of 3 nm.

7. The security element according to at least one of claims 1 to 6, characterized in that the color-shifting thin-film structure is applied to an embossing lacquer layer provided with a relief structure.

8. The security element according to claim 7, characterized in that the relief structure forms a diffractive structure, in particular a hologram, a holographic grating image or a hologram-like diffraction structure, an achromatic structure, in particular a matte structure, or a micromirror arrangement or a blaze grating with a sawtooth-like groove profile or a Fresnel lens arrangement, or a combination of two or more of the aforementioned structures.

9. The security element according to at least one of claims 1 to 8, characterized in that the security element is a security thread, a security strip, a patch, or a label. 10.Data carrier, in particular value document, with a security element according to one of claims 1 to 9.

11. Method for producing a security element according to one of claims 1 to 9, in which an absorber layer, a dielectric spacer layer and a reflection layer are applied to a substrate, and in which an ultra-thin adhesion promoter layer with a thickness of between 1 nm and 10 nm is applied, in particular sputtered, between the dielectric spacer layer and the reflection layer.

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